On the left, you can see that there’s only one competitive seat currently held by Republicans that’s in a Democratic leaning state: That’s in Maine, where Susan Collins is running for re-election. On the right, everywhere there’s a blue arrow, there’s a seat either held by a Democratic incumbent or being vacated by a Democrat that’s in a Republican state. Plus there are a bunch of seats currently held by Democrats in states where the vote totals tend to be close.

So it’s as simple as that: Senate Democrats are in trouble because this time around they hold a lot of seats in states with unfriendly terrain.